This invention relates to the drying and gasification of carbonaceous solids and is particularly concerned with drying an aqueous slurry of coal and the subsequent gasification of the dried coal.
Run-of-mine coal or similar carbonaceous solids will normally contain from about 5 to about 40 weight percent moisture depending upon the type of coal and the geographical area from which it is mined. It is normally desirable to remove this moisture or dry the solids before they are used as fuel to generate steam or otherwise produce heat, or before the solids are used as a feed to liquefaction, gasification, pyrolysis and similar processes wherein the carbonaceous feed material is converted into synthetic liquids and/or gases. Conventional methods for drying coal normally consist of contacting the coal or similar carbonaceous solids with a hot gas to vaporize the water thereby converting it to steam, which is ordinarily vented to the atmosphere. The hot gas may be air, nitrogen, or a similar gas that has been heated to a relatively high temperature. Since the resultant steam is vented to the atmosphere, the energy used to heat the gas is wasted and the drying process is inefficient. In some cases the gas used to dry the coal will be a flue gas produced by combusting a gaseous, liquid or solid fuel. If a flue gas is utilized to vaporize the water, it may contain undesirable constituents such as sulfur dioxide produced when the fuel is combusted and expensive scrubbing equipment may be needed to treat the flue gas after it has contacted the coal in order to prevent undesirable atmospheric emissions.
The inefficiency of drying coal and similar carbonaceous solids becomes more severe in catalytic gasification processes where the coal is impregnated with a catalytically active material prior to injection into the gasifier. The impregnation is normally carried out by mixing the coal with an aqueous solution of the catalyst and the resultant mixture is then dried. In such cases large amounts of heat are required to vaporize the water in the mixture and the resultant steam is vented to the atmosphere and its heat energy lost to the process. In noncatalytic gasification processes, it may be desirable to slurry the feed coal with water, pump the entire slurry to gasifier operating pressure and inject it into the gasifier thereby avoiding the use of complex lock-hopper systems to pressurize dry solids. Heat inefficiency, however, is still a problem in this method since the energy that would normally be utilized to dry the slurry prior to injection into the gasifier must now be supplied directly to the gasifier.
In both catalytic and noncatalytic gasification processes where coal is reacted with excess steam, the resultant raw product gas will contain unreacted steam which must be condensed and removed before the product gas is subjected to further processing. The condensed steam is sour water and contains hydrogen sulfide, ammonia and other impurities that are produced during the gasification step. This sour water must be stripped to remove a portion of these impurities and the stripped sour water then sent to wastewater treatment facilities to further purify the water before it can be reused or placed into the environment. These stripping and wastewater treatment steps are quite costly but are required in almost all steam gasification processes.